1. Field of the Invention
The invention relates generally to differential amplifiers and, in particular, to amplifiers providing balanced, or equal and opposite outputs when one input is driven.
2. Description of the Prior Art
Conventional differential amplifiers as illustrated in FIG. 1, do not produce equal amplitude outputs when one input is driven and the other input is AC grounded. Specifically, differential amplifier DA, having gain K, as inputs 13 and 16 and outputs 14 and 15. Input 12 is coupled 15 to a voltage generator providing a voltage V.sub.in and output 13 is grounded through capacitor 16. Inputs 12 and 13 are also properly DC biased. As a result, the voltage available at output 14 is equal to [K/(1+d)]V.sub.in and the voltage available at output 15 is equal to -KV.sub.in. Therefore, noninverted ouptput 14 has a magnitude which is less by amount KdV.sub.in /(1+d) than the magnitude of inverted output 15.
FIG. 2 illustrates a circuit diagram for a conventional differential amplifier. In general, a voltage V.sub.in is applied to the base of transistor Q11 and the base of the transistor Q12 is grounded through bypass capacitor C.sub.BP. In this configuration transistor Q12 functions as a common-base amplifier and transistor Q11 functions as a voltage follower. Considering the emitter of transistor Q12 as AC grounded, the gain G11 of stage 11 including transistor Q11 is related to the collector impedance which is equal to resistor R.sub.c1, and the emitter impedance which is equal to the series combination of emitter resistance R.sub.e1 and the parallel combination of resistor R.sub.e2 and resistor R.sub.cm where R.sub.e1 =R.sub.e2 =R.sub.e. Therefore, the gain G.sub.11 of stage 11 is given by formula 1 (see FIG. 4) where the parallel combination of emitter resistor R.sub.e2 and resistor R.sub.cm is given by formula 2. Substituting formula 2 into formula 1 illustrates that gain G.sub.11 of stage 11 is equal to the ratio of ##EQU1## since transistor Q12 is a common-base stage (see formula 3). As the value of resistor R.sub.cm approaches infinity, gain G.sub.11 approaches -R.sub.c /2R.sub.e. Therefore, resistor R.sub.cm, due to its parallel relationship with resistor R.sub.e2, causes gain G.sub.11 to be greater than this limit value.
This must be compared with the gain of stage 12. Stage 12 includes transistor Q12 in a common base mode with the emitter functioning as a summing junction. Thus, all currents into the emitter will be summed and appear at the collector. When considering transistor Q12, transistor Q11 is regarded as a voltage follower with the emitter resistors converting the emitter voltage of transistor Q11 into a current for application to the emitter of transistor Q12. The emitter current I.sub.e (Q11) of transistor Q11 is as shown in formula 4. However, not all of this current reaches the emitter of transistor Q12 because some of the current is shunted to ground by resistor R.sub.cm. As a result, the current I.sub.e (Q12) into the emitter of transistor Q12 is shown by formula 5. As shown in formula 6, this reduces current I.sub.e (Q11) applied to the emitter of Q12 and, therefore, the gain of G.sub.12 is reduced by an amount equal to R.sub.e /(R.sub.e +R.sub.cm). The conclusion is that the magnitude of gain G.sub.11 in stage 11 is always greater than the magnitude of gain G.sub.12 through stage 12 as illustrated in formula 7.
Another conventional configuration of a differential amplifier is shown in FIG. 3. Following a similar pattern of analysis as indicated above with regard to FIG. 2, gain G.sub.21 for stage 21 including transistor Q21 is shown by formula 8 and gain G.sub.22 for stage 22 including transistor Q22 is shown by formula 9. Once again, the magnitude of gain G.sub.21 of stage 21 is greater than the magnitude of the gain G.sub.22 of stage 22.
In the classical circuits illustrated in FIGS. 2 and 3, the additional gain to the V.sub.0 -output stage is due to an extra emitter current component. The additional emitter current component of the circuit in FIG. 2 is due to the emitter resistor R.sub.cm, while the additional emitter current component in the circuit in FIG. 3 is due to resistor R.sub.b1. To eliminate these components, resistor R.sub.cm, R.sub.b1 and R.sub.b2 must become infinitely large. However, this is very difficult to achieve at high frequencies such as frequencies in the range of 300 MHz.